Method for Forming Self-Assembled Mono-Layer Liner for Cu/Porous Low-k Interconnections

ABSTRACT

A method for fabricating an integrated circuit comprises forming a low-k dielectric layer over a semiconductor substrate, etching the low-k dielectric layer to form an opening, and treating the low-k dielectric layer with a gaseous organic chemical to cause a reaction between the low-k dielectric layer and the gaseous organic chemical. The gaseous organic chemical is free from silicon.

This application is a divisional of U.S. patent application Ser. No.12/509,039, filed on Jul. 24, 2009, and entitled “Method for FormingSelf-Assembled Mono-Layer Liner for Cu/Porous Low-k Interconnections,”which is a continuation of U.S. patent application Ser. No. 11/509,498,filed on Aug. 24, 2006, and entitled “Self-Assembled Mono-Layer Linerfor Cu/Porous Low-k Interconnections,” which claims the benefit of U.S.Provisional Patent Application Ser. No. 60/809,754, filed on May 31,2006, and entitled “Self-Assembled Mono-Layer Liner for Cu/Porous Low-kInterconnections;” the above applications are hereby incorporated hereinby reference in their entireties.

TECHNICAL FIELD

This invention relates generally to integrated circuits, and moreparticularly to the design and formation methods of interconnectstructures of the integrated circuits.

BACKGROUND

As the semiconductor industry introduces new generations of integratedcircuits (IC's) having higher performance and greater functionality, thedensity of the elements that form the integrated circuits is increased,and the dimensions, sizes and spacings between the individual componentsor elements are reduced. While in the past such reductions were limitedonly by the ability to define the structures photo-lithographically,device geometries having even smaller dimensions created new limitingfactors. For example, for any two adjacent conductive paths, as thedistance between the conductors decreases, the resulting capacitance (afunction of the dielectric constant (k) of the insulating materialdivided by the distance between conductive paths) increases. Thisincreased capacitance results in increased capacitive coupling betweenthe conductors, increased power consumption, and an increase in theresistive-capacitive (RC) time constant. Therefore, continualimprovement in semiconductor IC performance and functionality isdependent upon developing materials that form a dielectric film with alower dielectric constant (k) than that of the most commonly usedmaterial, silicon oxide, in order to reduce capacitance. As thedimensions of these devices get smaller and smaller, significantreduction in capacitance into the so-called “ultra low-k” regime isrequired.

New materials with low dielectric constants (known in the art as “low-kdielectrics”) are being investigated for use as insulators insemiconductor chip designs. A low dielectric constant material helps toenable further reductions in the integrated circuit feature dimensions.In conventional IC processing, SiO₂ was used as a basis for thedielectric material, resulting in a dielectric constant of about 3.9.Advanced low-k dielectric materials have dielectric constants belowabout 2.7. The substance with the lowest dielectric constant is air(k=1.0). Therefore, porous dielectrics are very promising candidates,since they have the potential to provide very low dielectric constants.

However, porous films have shortcomings. Poor time-dependent dielectricbreakdown (TDDB) performance has become a severe problem as a result ofthe seriously deteriorated barrier integrity. FIG. 1 illustrates aconventional interconnection formation scheme. A first copper line 4 isformed in a low-k dielectric layer 2. An etch stop layer 5 is formed onlow-k dielectric layer 2. A second copper line 12 is electricallycoupled to copper line 4 through a via 14. The second copper line 12 andvia 14 are formed in a porous low-k dielectric layer 6. A diffusionbarrier layer 10 is formed over sidewalls of the trench opening and viaopening, in which copper is filled to form second copper line 12 and via14. As low-k dielectric layer 6 is porous, the material in diffusionbarrier layer 10 may penetrate into pores that are exposed on sidewallsof the via opening and trench opening, thus causing clouding effects,which will adversely affect the subsequent interconnection formationprocesses. To solve this problem, a dielectric pore sealing layer 8 isformed on exposed surfaces of low-k dielectric layer 6 in the trench andvia openings to seal the pores. Dielectric pore sealing layer 8,however, typically has a higher k value than low-k dielectric layer 6has. The RC delay in the interconnect structure is thus increased. Inaddition, the adhesion of dielectric pore sealing layer 8 and diffusionbarrier layer 10 is not satisfactory, and delamination may occur in thesubsequent chemical mechanical polish process. Furthermore, dielectricpore sealing layer 8 is not conductive, and thus the portion on aninterface 16, which is exposed through the via opening, needs to beremoved by an additional liner removal step.

Accordingly, a method that maximizes the benefit of low-k dielectricswhile reducing the effects of their porous properties is needed.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, a method forfabricating an integrated circuit comprises forming a low-k dielectriclayer over a semiconductor substrate, etching the low-k dielectric layerto form an opening, and treating the low-k dielectric layer with agaseous organic chemical to cause a reaction between the low-kdielectric layer and the gaseous organic chemical. The gaseous organicchemical is free from silicon.

In accordance with yet another aspect of the present invention, a methodfor fabricating an integrated circuit comprises forming a low-kdielectric layer over a semiconductor substrate, etching the low-kdielectric layer to form an opening, and treating the low-k dielectriclayer with an organic chemical free from silicon. A molecular weight ofthe organic chemical is greater than about 60 g/mole.

In accordance with yet another aspect of the present invention, a methodfor forming an integrated circuit comprises forming a dielectric layercomprising porogen material, curing the dielectric layer to remove theporogen material to create pores in the dielectric layer, etching anopening in the dielectric layer, exposing the dielectric layer to anorganic gas to form a thin sealant layer in the opening on a sidewall ofthe dielectric layer, and forming a metal in the opening.

The advantageous features of the present invention include a reducedeffect of RC delay, improved adhesion between the pore sealing layer andthe diffusion barrier layer, and a saved liner removal step.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional interconnect structure including alow-k dielectric material;

FIGS. 2 through 7 are cross-sectional views of intermediate stages inthe manufacture of a preferred interconnect structure, wherein a dualdamascene process is illustrated;

FIG. 8 illustrates a cross-sectional view of a single damascenestructure in addition to the structure in FIG. 7; and

FIG. 9 illustrates a comparison of breakdown electrical fields of apreferred interconnect structure embodiment and a conventionalinterconnect structure embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

A novel method for forming a low-k dielectric layer and a correspondinginterconnect structure is provided. The intermediate stages formanufacturing the preferred embodiment of the present invention areillustrated. Variations of the preferred embodiments are then discussed.Throughout the various views and illustrative embodiments of the presentinvention, like reference numbers are used to designate like elements.

FIG. 2 illustrates a starting structure having a conductive line 22formed in a dielectric layer 20. Conductive line 22 and dielectric layer20 are over a semiconductor substrate (not shown), which is preferably asilicon substrate having semiconductor devices formed thereon.Conductive line 22 is preferably a metal line comprising copper,tungsten, aluminum, silver, gold, alloys thereof, compounds thereof, andcombinations thereof. Conductive line 22 is typically connected toanother underlying feature (not shown), such as a via or a contact plug.Dielectric layer 20 may be an inter-layer dielectric (ILD) layer or aninter-metal dielectric (IMD) layer, and preferably has a low k value.

An etch stop layer (ESL) 24 is formed on dielectric layer 20 andconductive line 22. Preferably, ESL 24 comprises nitrides,silicon-carbon based materials, carbon-doped oxides, and combinationsthereof. The preferred formation method is plasma enhanced chemicalvapor deposition (PECVD). However, other commonly used methods such ashigh-density plasma CVD (HDPCVD), atomic layer CVD (ALCVD), and the likecan also be used. In an exemplary embodiment wherein ESL 24 comprisessilicon nitride or silicon carbide, the formation is preferablyperformed in a chamber, in which gaseous precursors such as silane(SiH₄) and ammonia (NH₃) are introduced and a chemical reaction occurs.

In alternative embodiments, dielectric layer 24 is a diffusion barrierlayer preventing undesirable elements, such as copper, from diffusinginto the subsequently formed low-k dielectric layer. In a more preferredembodiment, dielectric layer 24 acts as both an etch stop layer and adiffusion barrier layer.

FIG. 3 illustrates the formation of a low-k dielectric layer 28, whichprovides insulation between conductive line 22 and overlying conductivelines that will be formed subsequently. Accordingly, low-k dielectriclayer 28 is sometimes referred to as an inter-metal dielectric (IMD)layer.

Low-k dielectric layer 28 preferably has a dielectric constant (k) valueof lower than about 3.5, and more preferably lower than about 2.5. Thepreferred materials include carbon-containing materials, organo-silicateglass, porogen-containing materials, and combinations thereof. Low-kdielectric layer 28 may be deposited using a chemical vapor deposition(CVD) method, preferably plasma enhanced CVD (PECVD), although othercommonly used deposition methods, such as low pressure CVD (LPCVD),atomic layer chemical vapor deposition (ALCVD), and spin-on, can also beused.

A first treatment, which preferably includes a curing process, is thenperformed. The curing process can be performed using commonly usedcuring methods, such as ultraviolet (UV) curing, eBeam curing, thermalcuring, and the like, and may be performed in a production tool that isalso used for PECVD, atomic layer deposition (ALD), LPCVD, etc. In anexemplary UV curing process, an ultraviolet radiator tool is utilized.The exemplary process conditions include a temperature of between about250° C. and about 450° C., a curing power of between about 3000 W andabout 6000 W, and a curing time of between about 300 seconds and about1500 seconds. The curing may be performed in a vacuum environment or inan environment containing process gases such as H₂, N₂, inert gases(including He, Ne, Ar, Kr, Xe, Rn), and combinations thereof. Thewavelength of the UV curing is preferably between about 200 nm and about400 nm.

The treatment serves the function of driving porogen out of low-kdielectric layer 28, thus improving its mechanical property. Pores willthen be generated in low-k dielectric layer 28. In the preferredembodiment, after the treatment, the porosity of low-k dielectric layer28 is preferably greater than about 15%.

FIG. 4 illustrates the formation of a via opening 30 and a trenchopening 32 in low-k dielectric layer 28. Photo resists (not shown) areformed and patterned over low-k dielectric layer 28 to aid in theformation of via opening 30 and trench opening 32. In the preferredembodiment, an anisotropic etch cuts through low-k dielectric layer 28and stops at ESL 24, thereby forming a via opening 30. Trench opening 32is then formed. Since there is no etch stop layer for stopping theetching of trench opening 32, etching time is controlled so that theetching of the trench opening 32 stops at a desired depth. Inalternative embodiments, a trench-first approach is taken, in whichtrench opening 32 is formed prior to the formation of via opening 30.ESL 24 is then etched through via opening 30, exposing underlyingconductive line 22.

In alternative embodiments, the previously discussed low-k dielectriclayer 28 may be replaced by a first low-k dielectric layer, an ESL onthe first low-k dielectric layer, and a second low-k dielectric layer onthe ESL. The ESL is used for etching trench opening 32. One skilled inthe art will realize the appropriate process steps.

In the preferred embodiment, low-k dielectric layer 28 contains silicon,oxygen and carbon. Carbon may be present in the form of methyl, ethyl,propyl, and the like. The low-k dielectric material may be schematicallyrepresented as PLK—Si—O—CH₃, wherein PLK represents a portion of theporous low-k dielectric material in low-k dielectric layer 28. Theetching process, however, typically causes the depletion of carbon fromlow-k dielectric layer 28, particularly on the surface of openings 30and 32. As a result, a surface portion of the respective low-kdielectric layer 28 may be represented by PLK—Si—O˜, wherein the symbol“˜” indicates dangling bonds. As such dangling bonds are not stable,they can easily form bonds with hydrogen to form OH terminals, and theresulting material on the surface of the low-k dielectric layer 28 isschematically represented by PLK—Si—OH. FIG. 4 schematically illustratessuch OH terminals on the exposed surfaces of low-k dielectric layer 28.

Referring to FIG. 5, a self-assembled mono-layer (SAM) formation processis performed to treat the exposed surfaces of low-k dielectric layer 28.In the preferred embodiment, the SAM formation process is performedusing plasma-assisted treating processes. Alternatively, thermaltreating processes are used. The process gases preferably include anorganic chemical containing methyl, ethyl, and/or propyl groups. Morepreferably, the organic chemical includes a methyl-containing gas. Theorganic chemical also preferably contains nitrogen, and/or a ring-typestructure with benzene rings, and is preferably free from silicon. Themolecular weight of the organic chemical is preferably greater thanabout 60 g/mole. In addition, carrier gases such as inert gases,nitrogen, hydrogen, ammonia, and combinations thereof are used. Theorganic chemical reacts with the surface portion of the porous low-kdielectric material, which is represented as PLK—Si—OH. The reactioncauses the bonds between oxygen and hydrogen to break, and the hydrogenions are replaced by methyl, ethyl, and/or propyl groups. The methyl,ethyl, and/or propyl groups are terminals, and thus the resultingmaterial on the surface of exposed low-k dielectric layer 28 stopsgrowing after the terminals are attached, resulting in a substantialmono-layer, which is denoted as layer 34. It should be noted that on thesurface of the exposed copper line 22, there are substantially no OHterminals, and thus the mono-layer 34 (SAM 34) is substantially notformed on copper line 22. Accordingly, such a process is self-assembled,hence the name self-assembled mono-layer (SAM) process. Also thedeposition process will stop after the terminals are formed, thus SAM 34is very thin. In an exemplary embodiment, SAM 34 has a thickness of lessthan about 50 Å, and is more likely between about 10 Å and 30 Å. In ananalysis of a sample that generated a Time-of-Flight Secondary Ion MassSpectrometry (ToF-SIMS) positive ion spectra, the presence of methylgroup terminals on the surface of low-k dielectric layer 28 has beenobserved.

Due to the formation of carbon-containing terminal groups, there is ahigh carbon concentration in a surface layer of low-k dielectric layer28. The concentration in the surface layer tends to be greater than ininner portions of low-k dielectric layer 28. In an exemplary embodiment,a surface layer with a thickness of 30 Å has a first average carbonconcentration higher than a second average carbon concentration of anunderlying portion with a thickness of 30 Å. In another exemplaryembodiment, the first average carbon concentration is greater than thesecond average carbon concentration by about three to about 15 percent.This is contrary to the prior art interconnect structure, wherein thecarbon concentration of the surface layer is typically lower than ininner portions due to carbon depletion. As methyl, ethyl, and propylhave greater volumes than hydrogen, the pores on the surface of low-kdielectric layer 28 are at least partially sealed.

An exemplary reaction equation is symbolically illustrated below. Pleasenote that this equation is only an example, and should not be used tolimit the scope of the present invention:

R₁—Si—RH+PLK—Si—O—H→PLK—Si—O—Si—R₁+RH_(x)  [Eq. 1]

wherein R₁ may be (CH₃)₃, and R may be a functional group containingnitrogen, and/or a ring-type structure with benzene rings, and R₁ ismethyl, ethyl, and/or propyl groups. An example of the above-expressionis:

(CH₃)₃—Si—RH+PLK—Si—O—H→PLK—Si—O—Si—(CH₃)₃+RH_(x).  [Eq. 2]

FIG. 5 illustrates a resulting structure after the SAM process. Aspreviously noted, the mono-layer is illustrated as layer 34. It is to benoted, however, that SAM 34 is so thin that it may not be visible, andthe existence of SAM 34 may need to be determined using equipment suchas time-of-flight secondary ion mass spectrometry (ToF-SIMS).

An optional UV curing is then performed on the structure shown in FIG.5. In the preferred embodiment, the UV curing may be performed in aproduction tool with the presence of hydrogen, nitrogen, inert gases,and combinations thereof. The preferred wavelength of the UV light ispreferably between about 200 nm and about 400 nm. The exemplary processconditions include a temperature of between about 250° C. and about 450°C., a curing power of between about 3000 W and about 6000 W, and acuring time of about 300 seconds to about 1500 s seconds.

FIG. 6 illustrates the formation of a barrier layer 38, which preventscopper from diffusing into low-k dielectric layer 28 and is preferablyformed of a material comprising titanium, titanium nitride, tantalum,tantalum nitride, and the like. It may be a single layer or have acomposite structure. As the pores on the surface of low-k dielectriclayer 28 are at least partially sealed, the clouding effects, whichcause the atoms of the diffusion barrier layer to penetrate into pores,is reduced. In an exemplary embodiment, the atoms of the diffusionbarrier layer penetrating into the SAM 34 are so few that the atomicpercentage of diffusion materials in SAM 34 is less than about 0.1%.

Referring to FIG. 7, via opening 30 and trench opening 32 are filledwith a conductive material, preferably copper or copper alloys. However,other metals and metal alloys such as aluminum, tungsten, silver andgold can also be used. A chemical mechanical polish is then performed tolevel the surface, forming a via 42 and a metal line 44.

The previously discussed embodiment illustrates the formation of a dualdamascene structure. SAMs can also be formed for single damascenestructures. FIG. 8 illustrates a SAM 50 formed in a single damascenestructure. One skilled in the art will realize the respective processsteps.

Due to the formation of SAM 34, the mechanical and electrical propertiesof interconnect structures are improved. SAM 34 has better adhesion tothe diffusion barrier layer than the conventional pore-sealing layerhad, and thus delamination is reduced. As SAM 34 will not be formed oncopper line 22, the liner removal step used in conventional processes toremove the dielectric pore-sealing layer on the copper line is saved.

Furthermore, by using the preferred embodiment of the present invention,electrical properties of the interconnect structures are improved. FIG.9 illustrates a comparison of a preferred interconnect structure formedwith a SAM and a conventional interconnect structure with a barrierlayer formed directly on a low-k dielectric layer. The data obtainedfrom the preferred embodiment are shown as hollow circles, while dataobtained from the conventional interconnect structure are shown as solidsquares. It is observed that the breakdown electrical field of thepreferred interconnect structure is about twice the breakdown electricalfield of the conventional interconnect structure. In addition, since SAMis very thin, the RC delay caused by SAM is substantially eliminated.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method for forming an integrated circuit, the method comprising:forming a dielectric layer comprising porogen material; removing theporogen material to create pores in the dielectric layer; forming arecess in the dielectric layer; forming a thin sealant layer in therecess on a sidewall of the dielectric layer; and forming a conductivematerial in the recess.
 2. The method of claim 1, wherein the removingthe porogen material comprises curing the dielectric layer.
 3. Themethod of claim 1, wherein the pores in the dielectric layercollectively comprise greater than 15 percent of the dielectric layer byvolume.
 4. The method of claim 1, wherein the forming the thin sealantlayer comprises exposing the dielectric layer to an organic gas.
 5. Themethod of claim 4, wherein the organic gas comprises methyl, ethyl,propyl groups or a combination thereof.
 6. The method of claim 4,wherein the organic gas comprises nitrogen or a ring-type structure withbenzene rings.
 7. The method of claim 4, wherein the organic gas is freefrom silicon.
 8. The method of claim 4, wherein the organic gascomprises a chemical with a molecular weight of greater than 60 g/mole.9. The method of claim 1, wherein the thin sealant layer is amono-layer.
 10. A method for forming an integrated circuit, the methodcomprising: forming a dielectric layer comprising porogen material;removing the porogen material to create pores in the dielectric layer bycuring the dielectric layer; etching an opening in the dielectric layer;forming a thin sealant layer in the opening on a sidewall of thedielectric layer by exposing the dielectric layer to an organic gas; andforming a metal in the opening.
 11. The method of claim 10, wherein thecuring the dielectric comprises ultra-violet (UV) curing, eBeam curing,or thermal curing.
 12. The method of claim 11, wherein the curingcomprises the UV curing, the UV curing comprising a temperature ofbetween 250° C. and 450° C., a curing power of between 3000 W and 6000W, and a curing time of between 300 seconds and 1500 seconds.
 13. Themethod of claim 10, wherein the pores in the dielectric layercollectively comprise greater than 15 percent of the dielectric layer byvolume.
 14. The method of claim 10, wherein the organic gas comprisesmethyl, ethyl, propyl groups or a combination thereof.
 15. The method ofclaim 10, wherein the organic gas comprises nitrogen or a ring-typestructure with benzene rings.
 16. The method of claim 10, wherein theforming the thin sealant layer further comprises using inert gases,nitrogen, hydrogen, ammonia, or a combination thereof.
 17. The method ofclaim 10 further comprising ultra-violet (UV) curing after exposing thedielectric to the organic gas.
 18. The method of claim 10, wherein theorganic gas is free from silicon.
 19. The method of claim 10, whereinthe organic gas comprises a chemical with a molecular weight of greaterthan 60 g/mole.
 20. The method of claim 10 further comprising forming abarrier layer in the opening.